Mechanistic Insights into the Inhibitory Role of Soil Humic Components in Iron (Oxyhydr)oxide Formation: From In Situ Kinetics to Molecular Thermodynamics

Due to the close spatial proximity and strong reactivity, soil humic components inevitably participate in iron (Fe) (oxyhydr)oxide formation, holding significant importance in contaminant immobilization, carbon cycling, and nutrient availability. Yet, the regulatory role of different humic components involved in the initial formation of Fe (oxyhydr)oxides is still lacking. In this study, we identified the characteristic formation periods of ferrihydrite (Fh), the initial phase of Fe (oxyhydr)oxides, through real-time monitoring of solution pH and in situ observations of precipitated Fh nanoparticles in the absence and presence of different humic components. The kinetics of Fh formation were quantified at micrometer and nanometer scales using Raman spectroscopy (RS) and atomic force microscopy (AFM), respectively. Results indicated that the extension of induction time, retardation of phase occurrence, and inhibition of nucleation rates for Fh formation were all dependent on the specific humic component with an order of fulvic acid (FA) > humic acid (HA) > humin (HM). Nanoscale data analysis revealed that the thermodynamic barrier to Fh nucleation increased by maximizing the interfacial free energy (γ) of the reaction system. Through molecular bonding quantification, AFM-based dynamic force spectroscopy (DFS) measurements demonstrated a linear relationship between Gibbs free energies (Δ<i>G</i>_b) of soil organic matter (SOM) binding to Fh and γ within the classical nucleation theory (CNT), linking heterogeneous nucleation barriers with organo-mineral bonding. This study is the first to provide in situ evidence of the inhibitory effects of soil humic components on the formation of Fe (oxyhydr)oxides and quantitatively establish that higher energy barriers to nucleation correlate with stronger organo-mineral bonding. This relationship suggests that good organic binders are good inhibitors for mineral formation, offering a novel perspective for predicting the formation and fate of soil minerals through the lens of organo-mineral binding free energies.